Radio distance measuring apparatus



4, 194?. G. v. ELTGROTH 2,416,741

RADIO DISTANCE MEASURING APPARATUS Filed Oct. 15, 1942 2 Sheets-Sheet l E RECEIVER TRANSMITTER AND DEMODULATOR SOURCE OF 3 MODULAT/NG VOL m 615 PHHSE @0MPAR4T0R l6 RECEIVER AND 2 m TRA/VflM/TTER DEMODULATUR nveni'or March 1947. s. v. ELTGROTH RADIO DISTANCE MEASURING APPARATUS 2 Sheets-Sheet 2 Filed Oct. 15, 1942 fiatented Mar. 4, ll'? UNITD RADIO DISTANCE MEASURING APPARATUS George V. Eltgroth, Towson, M11, assignor to Bendix Aviation Corporation, South Bend, End, a corporation of Delaware Application October 15, 1942, Serial No. 462,167

.each of the two points and the mode of operation is about as follows: A radio transmitter at the reference location is amplitude modulated with energy from a low frequency source of alternating current and the energy transmitted is received and demodulated at a distant point; the output of the remote demodulator is now em ployed to amplitude modulate the utput of an associated transmitter working on a carrier frequency spaced from that of the reference transmitter, and signals from the remote transmitter are received and demodulated at the reference location. The linear separation between the two points is now determined by measurement of the phase relation between the outgoing and incoming modulation at the reference location, as this phase shift is a function of the total distance traversed by the radio waves.

A complete system was ,constructed in accordance with the teachings of the above patents employing amplitude modulation and placed in operation utilizing a modulation frequency of 1000 cycles per second. During the tests, it was found that the distance indications were sometimes in error by distances of 12 miles or more, depending upon the adjustment and tuning of the receivers employed, and this error was ultimately traced to the phase shift of the modulation envelope occurring within the receiver, which was as much as 50 degrees when the receiver was tuned exactly to resonance and varied rapidly with slight mistuning of the receiver. Systems of this nature find their primary application in the navigation of aircraft and it is well known that the demands of other operations on the time of the operator are such as to preclude the use of equip ment requiring very precise adjustment to secure error free indications. Consequently, the use of systems of the above type has been severely limited with the resultant loss of a potentially valuable aid to navigation.

Phase shift between the modulation envelope of the input current to a parallel resonant cir- 4 Claims. ((31. 250-1) cuit and the modulation envelope of the voltage appearing across the parallel resonant circuit is due to the fact that the side bands are shifted in phase with respect to the carrier. When a wave modulated with a frequency equal to T o% 0f the carrier frequency is impressed on a parallel resonant circuit having a Q of 200, the high frequency sideband is caused to lag its normal position by approximately 8 degrees, while the low frequency sideband is caused to lead by the same amount and the modulation envelope of the voltage across the circuit is therefore shifted in phase from thatof the modulation envelope of the input current by 8 degrees. The average receiver may have six cascaded tuned circuits each contributing to the overall phase shift which is thus 48 degrees with the receiver tuned accurately to resonance and varies rapidly with even slight mistuning as the phase relations between the sidebands and carrier are altered. While it is theoretically possible to overcome the above defect by the use of low Q circuits in the receivers, in practice the required Q is so low that an economically excessive number of tuned circuits is required to obtain the necessary selectivity and gain.

One of the principal objects of the invention is to provide a new and novel radio distance measuring system havin improved accuracy.

Another object of the invention is to provide a radio distance measuring system in which the operation is simplified, and a high degree of skill is unnecessary.

Still another object of the invention is to provide a radio distance measuring system in which accurate tuning of .the transmitting and receiving apparatus is not absolutely necessaryjfor accurate indications.

The above objects and advantages of the in vention are accomplished by making use of frequency modulation in the transmitters at the reference and remote points and providing the receivers at these locations With frequency modulation detectors. I have found that the presence of phase shift in the tuned circuits through which frequency modulated energy is passed is Without effect on the phase of the. demodulated output and produces only amplitude distortion in this energy. Further, since the amount of phase shift produced during modulation, for example, of a carrier wave 75 kilocycles on either side of the mean carrier frequency by a 1000-cycle Wave is about 4300 degrees which is far in excess of the 50 degrees or so contributed by the receiver tuned circuits. the distortion introduced by phase shift in the receiver tuned circuits thus can never rise to a magnitude that will appreciably impair the accuracy of the phase indication.

Other objects and advantages of the invention will in part be described and in part be obvious when the following specification is read in conjunction with the drawings in which:

Figure 1 is a block diagram illustrating the elements of the operation of the system described.

Figure 2 is a schematic diagram of the apparatus situated at the reference location or point at which the transmission originates.

Referring to Figure 1, the transmitter l is connected to the radiating antenna 2 and the output of the transmitter is modulated by the output of the modulating source 3. The radiated energy is intercepted at the mobile location by the receiving antenna d whose output is impressed on the receiver and demodulator and the output of the demodulator is in turn utilized to modulate the output of the transmitter 6, preferably operating on a frequency different from that of transmitter i, and this output is then radiated from transmitting antenna l. The energy thus relayed is received by the antenna 8 and conveyed to the input of receiver and demodulator 9 whose output is connected to phase comparator Iii. The phase comparator It] is also connected to the source of modulation voltage 3 and indicates the phase difference between the incoming modulation envelope and the outgoing modulation envelope. Inasmuch as this phase diiTerence is determined by the time required for the round trip of the electromagnetic energy between these two points, the linear separation may be readily determined. When a modulating frequency of 1000 cycles per second is employed, a phase shift of 360 degrees exists when the two sets of apparatus are separated by 93 miles and consequently each degree indication on the phase meter corresponds to a distance increment of approximately one quarter mile. The system in this elementary form ha already been described in the earlier mentioned patents.

Figure 2 illustrates and the following description describes in detail the apparatus located at the reference, and mobile locations. The heaters associated with the emissive cathodes of the vacuum tubes employed in this apparatus have been omitted from the drawings for the purpose of simplicity and clarity.

Turning to Figure 2 there is shown the oscillator tube H with the cathode I2 connected to a tap on the oscillator coil 13. One end of coil 3 is grounded while the other end is connected to the control grid hi of oscillator tube I I through the condenser I5. A direct current path between control grid i4 and cathode I2 is established through the grid leakresistor It. The mean frequency of oscillation of this portion of the circuit is determined by the adjustment of the variable condenser I! connected across the ends of inductance iii, The space charge grid l8 of oscillator tube II is connected to the positive terminal of the direct current source through the dropping resistor I9 and grid I8 is grounded for radio frequency potentials by the bypass condenser 2! connected between this grid and ground. The two grids and the cathode of oscillator I! cooperate to produce oscillations in the inductance I3 and capacitor H after the well known manner of the Hartley oscillator. These oscillations vary the electron current arriving at the anode 22 which is connected to the positive terminal of source 20 through the parallel circuit of resistance 23, capacitance 24 and inductance 25, and voltages at the oscillation frequency of the inner element assembly appear across the loaded tuned output, circuit.

The condenser 26 connected between the anode 22 and the control grid 27 of the power amplifier 28 transfers the output energy from the oscillator to this point. Amplifier 28 is biased by the grid current flowing through the combination of resistance 29 and choke 39 connected between control grid 27 and ground. Cathode 3! is connected to ground and the space charge grid 32 is connected to the positive terminal of source 20 through the dropping resistor 33. Grid 32 is grounded for radio frequency potentials by the bypass condenser 34 connected between this grid and ground. Anode 35 is also connected to source 29 through the output load circuit including inductance 36 paralleled by capacitor 3?, and the energy developed in this circuit is transferred to the secondary winding 38 which is connected to the radiating antenna 39 in series with the antenna tuning capacitors iii.

Modulation of the oscillator output is eifected by the circuit including modulator tube 4!. A portion Of the energy from the tuned oscillator circuit is taken oif by the series combination of resistance 42 and capacity 133 connected between the ungrounded terminal of inductance l3 and ground and the voltage developed across capacitor (it is impressed on the control grid 44 which is connected to the junction between resistor 42 and capacitor 4 3. The cathode 45 of tube 4| is connected to the suppressor grid 46 and both elements are returned to ground through the parallel combination of resistance 4'1 and capacitor 418. Anode 49 of tube 4! is connected to the ungrounded terminal of inductance !3 through the capacitor 58 and is connected to the positive terminal of source 28 through the choke 5i and resistance 52. The low potential end of resistance 52 is grounded by the bypass capacitor 53 connected to ground therefrom. Also situated in tube 4! is the space charge grid 54 which is connected to the low potential end of resistor 52 through the dropping resistor 55 and is maintained at ground potential for radio frequency currents by the capacitor 56 connected between the grid end of resistor 55 and ground. Modulating potentials for the tube '4! are derived from the modulation source 5'! having one terminal grounded and the other terminal connected to control grid i l through the isolating choke 53 having high impedance at the oscillator frequency. Source 51 preferably has low direct current resistance to afford a grid return for tube vM and is shunted by the capacitor 59 to prevent the appearance of radio frequency potentials across the output terminals thereof. Output energy from source 51 is also impressed on a set of input terminals of the phase meter 69 via the leads 5!.

In operation, the voltage across the capacitor 43 lags the voltage across the oscillator tuned circuit and, since the plate current of modulator ll is in phase with the impressed grid signal, this tube draws a lagging current from the tuned circuit. The magnitude of the lagging current drawn is controlled by the bias on control grid 68 and the modulator tube M consequently appears to the tuned circuit as a variable shunt inductance whose variations are controlled by the output of modulation source 51, producing variations in the output frequency of oscillator II controlled by the Waveform of the modulating potential. The frequency modulated output of oscillator II is amplified by power amplifier 28 and then radiated from the antenna 39. The resistance 23 across the oscillator output circuit broadens the response of the tuned circuit to prevent the introduction of distortion by excessive phase shift variations in this circuit during the modulation cycle. In the output circuit of amplifier 28, the same function is achieved by coupling the secondary 38 to the primary 36 closely enough to provide the proper reflected resistance across pri mary 36. The energy radiated by the antenna 39 is received at the craft whose location is to be determined, demodulated and employed to frequency modulate the emission of another transmitter on the craft operating on a frequency different from that of the transmitter just described. This reradiated energy is intercepted by receiving antenna 62 and impressed on the transformer primary 63 which then transfers it to the secondary winding E l tuned by the variable capacitor 65. One terminal of secondary 6:; is connected to ground, while the other is connected to the control grid 68 of the radio frequency amplifier tube 6?. Tube 6'? is provided with a cathode 58 which is connected to ground through the parallel resistance Iii and capacitor II to provide operating bias for control grid 85. The space charge grid I2 is connected to the positive terminal of the direct current source lit by the dropping resistor I I and is grounded for radio frequency currents by the bypass capacitor it connected between this grid and ground, while the anode Iii is connected directly to source '53 through the primary winding I? of the interstage coupling transformer. Alternating current energy in the anode circuit of tube 6i is transferred to the secondary winding I8 of the interstage coupling transformer by virtue of its coupling with primary winding TI and impressed on the signal control grid IQ of the mixer tube 883. The voltage developed across winding It is increased by tuning this winding with the variable condenser 83 and the response of the tuned circuit is broadened by the resistor 82 in parallel with condenser 85 to insure the passage of all the necessary sidebands. The cathode 83 of tube Bil is connected to ground through the paralleled resistor 84 and capacitor 85 to provide the necessary operating bias for the signal grid it of the mixer tube 88. The inner grid 86 of mixer tube 86 is connected to the control grid Hi5 of the oscillator tube IiiI which is connected as a Hartley osciliator with cathode i112 connected to the tap on the oscillator inductance I83 having one terminal grounded and the other connected to control grid I653 by the capacitor Illa. The oscillator grid bias is provided by the grid current flowing through the grid leak resistor Ind connected between control grid I66 and the cathode I62, while the oscillator frequency is determined by the value of the variable capacitor Idfi connected across the coil,I03. The electrical energy necessary to the operation of the oscillator is supplied from the source I3 through the dropping resistor 536 to the anode IB'I which is grounded for radio frequency currents by the bypass capacitor iilfi. Signal grid I9 of tube 8% is surrounded by the space charge grid 88 energized from source I3 through the resistor 8i and grounded for radio frequency by the bypass condenser 39 connected from grid 88 to ground and the signals on grids it and 8B coact on the electron stream from the cathode 83 to provide not only the original input frequencies, but also beat frequencies in the current flowing to the anode 9B which is connected to the positive terminal of source I3 through the parallel circuit of transformer primary SI, tuning capacitor 92 and loading resistor 93. The tuning of this current serves to select the desired beat frequency in the output of mixer 88 in the manner well known to those familiar with superheterodynes.

The output energy from the mixer 8% is transferred to the control grid 95 of the limiter tube 94 by transformer action between primary Winding 9| and the secondary winding 96 tuned by the capacitor 91 parallel by the loading resistor 98, and the control grid 95 derives its operating bias from the grid current flowing through the resistor 99 connected between the low potential end of winding 95 and ground. The dynamic characteristics of the limiter are largely fixed by the capacitor III] connected across resistor 99 and ground. The cathode III and suppressor H2 of the limiter tube 94 are connected directly to ground, and the space charge grid I! 3 is energized by energy obtained from source It through the resistor I I I connected at its low potential end to the grid I I3 and the grounded bypass capacitor I I5. The output of limitersd appearing across the anode load circuit formed by the transformer primary IE6 paralleled by tuning capacitor Ill and the loading resistor H8 connected to the positive terminal of source 13 and to the anode I25 is substantially constant regardless of frequency variations occurring during the modulation cycle and changes in the amplitude of the input signal because of the self-regulating action of the voltage developed across resistor 99. Signal voltages also appear in the center tapped secondary winding II 9 which is coupled to primary winding H6 and Winding H9 is tuned to resonance with the signal by means of the tuning capacitor I28 shunted with the loading resistor i2I. The two ends of winding I59 are connected respectively to anode I22 and anode I23 of the discriminator rectifier I24 while the center tap of winding I I 9 is connected to the anode I25 of limiter 94 by the blocking capacitor I26. The cathode I2'I associated with anode I23 is connected directly to ground and cathode I28 associated with anode 522 is connected to ground through the two serially connected resistors I29 and I353 having their junction point connected to the center tap of winding I I9. Cathode I28 is grounded for currents of the signal frequency by the series resonant circuit tuned to the selected operating beatifrequency in the output of mixer 86 consisting of the inductance I3I and capacitor I32 connected between this point and ground. In a manner familiar to those skilled in the art, variations in the frequency of the signal impressed on primary winding I I6 give rise to corresponding variations in the voltage between cathode I28 and ground; since the signal is frequency modulated by the output of the modulator source 5'1, there therefore appears at cathode I28 an output volt age having the same frequency and substantially the same waveform as the source 5?. However, this voltage is displaced in time or phase by an amount dependent on the time required for the signal round trip between the reference location and the unknown location of the craft bearing the relay equipment, and this time may thus be determined by the measurement of the phase displacement. Y

The output power from the discriminator rectifier I24 is insufficient for the operation of many 7 phase measuring devices so it is transmitted through condenser I33 to the potentiometer I34 having. an adjustable tap connected to the control grid I3-5 of the audio frequency amplifier I36. The cathode I31 of amplifier I35 is connected to ground through the cathode bias resistor I38paralleled by the bypass capacitor I39 and the anode I40 is connected to the positive terminal of source I3 through the anode load resistor MI. Amplification takes place in this stage in a manner well known and the voltage appearing across the anode load resistor MI is impressed on control grid. I42 of the power amplifier I43 via the coupling capacitor I44 whose grid terminal is grounded through grid leak resistor I45; fier cathode I46 and the suppressor I41 are connected together and to ground through the familiar combination of resistance I 88 shunted by capacitor I49, the space charge grid I 53 being connected directly to the positive terminal of source I3 while the output transformer primary winding I5I is connected between this point and the anode I52. If desired, primary I5I may be shunted by a capacitor I53 to reduce the intensity of whatever harmonics may be present in the output of the power amplifier M3. The secondary winding lid is coupled to primar winding IEI and is connected to a second set of input terminals on the phase meter by the connecting leads I55. have the structure of any of the many well known types, is thus connected to the output of the modulation source 51- and. to the output of the frequency modulation receiver including the power amplifier I43 and continuously indicates the phase relation existing between the voltages appearing at these points.

The apparatus employed on the craft whose location is to be determined is identical with that shown in Figure 2 save that phase meter 69 and the modulation source 51 are omitted and the output winding I54 has one terminal connected to ground and the other to the junction between capacitor 59 and the isolating choke 53. At the mobile or unknown location, therefore, the output of the receiver is used to modulate the emission of the transmitter which operates on a frequency different from that radiated by the transmitter at the reference location, and the emanation of the transmitter at the mobile location is received by the receiver at the reference location. The demodulated output of the receiver is then compared in phase with the modulatingvoltage impressed on the frequency modulator to indicate the separation of the two transmitters.

As an alternative to the apparatus above outlined at the mobile location, the signal from the reference location may be received, amplified, translated to a new position in the frequency spectrum by modulation with a fixed frequency oscillator, again amplified and then i e-radiated to the reference location. In this system, all audio frequency circuits are eliminated at the mobile location thereby avoiding the phase shift which may occur therein.

In operation, the transmittingv frequency is controlled by the adjustment of the variable capacitor I1, and capacitors 24, 3'1, and do are adjusted to secure maximum radiated power. In the receiver, capacitors 92, 9'1, Ill, and I 28 are adjusted for maximum gain at a predetermined fixed frequency termed the intermediate frequency and capacitors and 8| are set to provide the best gain and selectivity at the signal frequency, which may vary. The output fre- Power ampli- Phase meter 69, which may quency of oscillator IUI is adjusted by varying capacitor I35 until the desired beat frequency is developed in the anode circuit of the mixer 80. With frequency modulation employed as outlined in the foregoing description, it is found that errors in receiver tuning adjustment do not shift the phase of the output from the audio amplifier circuits with respect to the modulation phase in the input signal and the indications obtained on phase meter 69 may be relied upon under all conditions to an accuracy determined only by the accuracy of the phase meter itself. The advantage over previously built structures employing amplitude modulation in which extremely careful and accurate adjustment of the receiver tuning controls was imperative if errors were to be held to values of less than 5 degrees is clear. In the system of the invention, the receivers may be tuned in the customary carefree manner, and so long as any signal is obtained from the receiver output at the reference location, the indication is the correct'one.

It will be obvious that many changes and modifications may be made in the invention without departing from the spirit thereof as expressed in the foregoing. description and. inv the appended claims.

I claim:

1. In a radio distance measuring system, means for radiating a frequency modulated carrier, means remotely located from said radiating means for receiving said frequency modulated carrier, transmitting mean connected to said receiving means radiating a frequency modulated carrier with a modulation characteristic controlled by the modulation characteristic of said first mentioned frequency modulated carrier and having a mean carrier frequency differing from that of said first mentioned carrier, means for receiving said second mentioned frequency modulated carrier, and indicating means responsive to the phase difference between the modulation characteristic of said first mentioned frequency modulated carrier and the modulation characteristic of said second mentioned frequency modulated carrier.

2. In a radio distance measuring system, a source of modulating potentials, a source of periodic energy, modulating means for varying the output frequency of said second mentioned source in accordance with said modulating potentials, means for radiating the variable frequency periodic energy produced by the action of said modulating means, means remotely located from said radiating means for receiving said radiated periodic energy and demodulating said energy to produce a replica of said modulating potentials, a second source of periodic energy, means for varying the output frequency of said second mentioned source of periodic energy in accordance with said modulation potential replica, means for' radiating the variable frequency periodic energy controlled by said modulation potential replica, means for receiving said replica controlled periodic energy, means for demodulating said second mentioned received energy, and indicating means responsive'to the phase difference between the output of said first mentioned source and the output of said demodulating means.

3. In a radio distance measuring. system, a source of modulating voltage, means for radiating a carrier controlled by the output of said source, selectively responsive receiving and demodulating means actuated by said carrier in which the phase relation between input modulation and output is independent of the tuning of said receiving and demodulating means with respect to the carrier frequency remotely located from said radiating means, means for radiating a second carrier controlled by the output of said receiving and demodulating means, selectively responsive receiving and demodulating means actuated by said second carrier in which the phase relation between input modulation and output is independent of the tuning of said receiving and demodulating means with respect to said second carrier frequency, and phase responsive means connected to the output of said second receiving demodulating means and to the output of said source of modulating voltage.

4. In a radio distance measuring system, a source of modulating potentials, means for radiating a frequency modulated carrier controlled by said modulating potentials, means remotely located from said radiating means for receiving said frequency modulated carrier, transmitting means connected to said receiving means radi- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,147,810 Alford Feb. 21, 1939 2,248,727 Strobel July 8, 1941 2,301,929 Budenbom Nov. 1'7, 1942 

